In carburetors for automobile engines having a main system for delivering main fuel to the venturi area of an air intake passage that passes through the carburetor main body, and a low-speed system for delivering low-speed fuel to the throttle valve area, the low-speed system is typically branched from the main system. In contrast, in carburetors for general-purpose engines, see e.g. Japanese Patent Application Laid-open No. 47-38218 and Japanese Patent Application Laid-open No. 55-69748, the two systems are separated and the low-speed system is made independent with respect to the main system; the advantages of which are the main fuel begins to flow early, and there is no back bleed phenomena in the low-speed system.
However, in carburetors for general-purpose engines, a small amount of fuel is handled in comparison with carburetors for automobile engines. As a result, the fuel flow rate that affects the engine performance or exhaust gas must be precisely adjusted. It is for this reason that in addition to providing a main jet for restricting the maximum fuel flow rate of the main fuel and a low-speed jet for restricting the maximum fuel flow rate of the low-speed fuel, an adjustment screw with a needle valve, such as that cited in the above-mentioned publications, is provided to allow the main fuel flow rate and the low-speed fuel flow rate to be separately adjusted.
In other words, the variability in the fuel flow rate fed to the engine is the combined variability in the dimensions of the main jet and the low-speed jet. The deviation in the fuel flow rate due to variability in these dimensions, and the difference in the required fuel flow rate due to variability in individual engines are corrected using an adjustment screw.
However, the fuel flow rate restriction and adjustment devices, such as those described above, separately restrict or correct the flow rate of fuel that flows respectively through the main system and the low-speed system. The disadvantages for the above-described conventional carburetors having such devices are that when the venturi diameter is increased in order to ensure high output, the time at which the main fuel begins to flow is delayed even in a type in which the low-speed system is made independent from the main system, a temporary fuel flow rate deficiency is also created because the low-speed fuel flow rate is limited to a set rate or less, and the transition from low-output operation to mid- or high-output operation cannot be carried out smoothly.